Solid state image sensors are well known. Commonly solid state image sensors are implemented in a CCD-technology or in a CMOS- or MOS-technology. Solid state image sensors find a widespread use in camera systems. A matrix of pixels comprising light sensitive elements constitutes an image sensor, which is mounted in the camera system. The signal of the matrix is measured and transformed to a so-called video-signal.
CCD-based camera systems have less noise fluctuations in the image compared to CMOS- or MOS-based camera systems. Therefore CCD-based camera systems are nowadays preferred in applications wherein a high image quality is required such as video or still camera applications. Due to the further miniaturization of the CMOS electronics technology, it is possible to realize complex CMOS- or MOS-based pixels as small as CCD-based pixels. It is a further advantage of CMOS- or MOS-based pixels that CMOS is a technology being widely offered whereas CCD-technology is rarely offered and is a more complex and expensive one.
Of the image sensors implemented in a CMOS- or MOS-technology, CMOS or MOS image sensors with passive pixels and CMOS or MOS image sensors with active pixels are known. An active pixel is configured with means integrated in the pixel to amplify the charge that is collected on the light sensitive element. Passive pixels do not have such means and require a charge-sensitive amplifier that is not integrated in the pixel but is connected with a line towards the pixel.
The use of space-variant visual sensors in image communication and processing is graining more and more attention as a simple and direct way of reducing the visual information transmitted and/or processed while preserving both high resolution and a wide field of view.
Several attempts have been made to make visual sensors which are compact and which provide good resolution at least in a central portion of the sensor array. One such sensor array is known from U.S. Pat. No. 5,166,511 and includes a central square CCD array and outer rings of radiation sensitive elements. This device has the disadvantage that there is a significant discontinuity between the central Cartesian array and the outer polar array. A similar device in CMOS technology is described in the article by Wodnicki et al. entitled “A foveated image sensor in standard CMOS technology”, Proc. Custom Integrated Circuits Conf pages 357–360, 1995. A further attempt has been made and an enlarged view of the central portion of the sensor array is shown in FIG. 1. As can be seen, radial lines of sensors end abruptly causing a local discontinuity in resolution. Although the transition between the central sensor array region and the outer region is better than previously mentioned examples, there are still local discontinuities which can affect picture quality.
U.S. Pat. No. 4,267,573 describes an interesting device in which the sensors are located on logarithmic spirals. However, at the center of the sensor array the sensor density approaches infinity. Hence, for the central region another solution has to be found, e.g. a hole is left in the middle where there are no sensors. U.S. Pat. No. 5,587,580 shows a polar array of sensor elements. However, how the central region is dealt with is not described. U.S. Pat. No. 5,712,729 shows a non-spatially variant array based on a hexagonal geometrical pattern. Non-spatially variant arrays are wasteful of sensors at large radii. Further, a hexagonal geometry also includes a local discontinuity at each apex.
The present invention has for its object to realize a spatially variant sensor array which is substantially free of local or global discontinuities.
Still a further object of the present invention is to provide an electronic camera with a spatial arrangement which may have a similar functionality to that of the human retina.